Big-Block Heads Shootout - The Big O Vs. The Big R

Oval ports and rectangular port heads

Two issues back, we subjected a 468-inch big-block Chevy to a variety of different oval-port cylinder heads, ranging from stock peanut-port-style heads to full CNC-ported versions (see “The Big O”). In that story, we attempted to dispel the myth that any powerful big-block combination must include rectangular-port cylinder heads. Equipped with the right oval-port heads, the 468 easily exceeded 600 hp on the engine dyno. Most of the oval-port heads tested in part 1 offered airflow that would support another 100 hp on the right combination.

Contrary to popular opinion, our combination was limiting the potential of the oval-port cylinder heads, not the other way around. It is true that all of the factory performance big-blocks sported rectangular-port heads back in the day, but much has changed since the first muscle car era. Not only does a good set of aftermarket oval-port heads outflow the factory rec-port stuff, it does so with reduced port volume. Big, lazy ports are ideally suited for neither performance nor street use, while smaller, efficient ports offer the best of both worlds. Nowhere was this more evident than in the fact that a couple of manufacturers chose to supply oval-port heads once again on this larger (and more powerful) 496 test engine.

A common stroker displacement, our 496 was the result of combining a 0.060-inch overbore with a 4.25-inch stroker crank. Our BBC rotating assembly came from the experts at Scat Enterprises. The BBC combination featured a 4340 forged steel crank combined with a set of matching 6.385-inch, I-beam rods. We chose a Scat crankshaft design specifically for our late-model, one-piece, four-bolt, Gen-6 block. The Scat crank and rods were combined with a set of forged pistons from JE. Offered as part of their SRP line up, the JE Pistons featured 18cc domes to produce a static compression ratio of 10.0:1 with typical 120cc combustion chambers.

The short-block was built not to maximize power production, but rather to demonstrate what is possible for street use in a performance driver. We wanted all of our testing to be run on pump gas, so we kept the static compression at a reasonable level. L&R Automotive was responsible for machining and balancing of the combination, while Total Seal came through with a set of performance rings to ensure proper sealing.

The displacement ensured that our 496 would be more powerful than the 468 used in part 1, but we hedged our bets with the installation of a wilder cam profile as well. Balancing the street/performance theme, we chose a small solid roller profile from Comp Cams. The 300BR-14 offered 0.652 lift, a 255/262 duration split and 114-degree LSA. To work in the Gen-6 block, the cam was teamed with a set of 0.300-tall solid roller lifters and a double roller timing chain.

Since the cam and timing chain were designed for a Gen-4 block, it was necessary to eliminate the factory cam retaining plate in favor of a traditional cam button. The swap also required the use of a custom front cover (PN217) from Comp Cams to provide the necessary room for the double-roller chain (factory Gen-6 covers must be run with a single-roller chain).

Prior to running the test engine, all of the heads were treated to flow bench testing to correlate the flow potential to the power gains. In truth, heads like the Brodix BB-3 Xtra O offered over 400 cfm--enough to support more than 800 hp. We were just scratching the surface or their potential with our 496 street/strip combination. After bench testing, we subjected the heads to port and chamber volume measurements. Since the compression ration is a function of the chamber volume, chamber size has a significant effect on the power curve (to the tune of 3-4 percent per point). Measurements indicated that our test heads varied greatly, from a low of 107cc to a high of 123cc. Bear this in mind when viewing the power numbers.

It is again worth mentioning that though this was originally to be a rec-port-only head test (to follow the oval-ports tested in part 1), some of the manufacturers supplied oval-port heads for this 496. Don't look down you noses at oval-port heads, as testing from both stories suggests that if anything, oval is the new square.

MORE PHOTOS

9 Our shortblock featured a 4340 forged steel crank and rods from Scat. The 4.250-inch stroker crank combined with the .60-over pistons to increase the displacement to 496 cubic inches.

10 JE pistons supplied the necessary forged slugs for our build up. The small-dome pistons were employed to keep the static compression ratio over 10.0:1, but the chamber volume of the heads tested varied from a low of 107 cc to a high of 123 cc (a change of more than 1 full point).

11 Proper sealing is important on any motor but critical on a test motor. Total Seal provided the necessary ring package to ensure that ever ounce of compression was used to move those pistons.

12 Milodon came through with a complete oiling system, including a pan, pick up and windage tray. The kit included a new oil pump, hardened pump shaft and main studs to properly locate the windage tray.

13 Combining our roller cam with a late-model, Gen-6 block necessitated use of a .300-tall solid roller lifters from Comp Cams. Comp also supplied a number of different sets of pushrods to work with all the different head combinations.

14 Wanting to increase power relative to the 468 used in part 1 to test to oval-port heads, we stepped up the cam for our 496. The Comp 300BR14 cam offered .652 lift, a 255/262 duration split and 114-degree LSA.

15 Since the solid roller cam was designed for a mark IV BBC, use in our Gen 6 block required not only removal of the cam retaining plate but also a dedicated front cover from Comp Cams. The two-piece, aluminum front cover provided the necessary room to run a double-roller timing chain on the Gen 6 stroker.

16 All heads for the test were secured using Fel Pro 1047 head gaskets and ARP head studs or bolts. The different heads required different length studs/bolts, but ARP supplied everything needed to complete our head test.

17 Comp Cams also came through with a set of 1.7-ratio, aluminum roller rockers.

18 Induction chores were handled by an Edlebrock Victor Jr. 454-R intake and Holley 950 HP carb. The lone exception was the AFR set that used a 454-O intake designed for oval-port heads.

19 We specified a spring package designed for the .650-lift roller cam, but three of the heads came equipped with insufficient spring pressure for the test. The stock heads, Summit iron and Edelbrock heads all required a valve spring upgrade. The stock heads received a spring upgrade from Comp Cams, while the Edelbrock and Summit heads were run with the spring package borrowed from the Brodix heads.

20 Prior to running on the engine dyno, all heads were tested on the Super Flow 600 airflow bench. Testing on the same day with the same fixtures allows for a direct comparison.